The invention has been devised primarily in the context of what are known as "rolling loop" motion picture projectors, although it is to be noted that the invention is not limited to mechanisms for use in optical projection or recording systems. U.S. Pat. No. 3,494,524 to Jones discloses the principle of a rolling loop film transport mechanism. Briefly, the Jones mechanism includes a rotor having gaps and a curved stator for guiding the film between the rotor and the stator. As the rotor rotates, successive loops of film are formed in the rotor gaps, causing the film to move through one frame length each time the loop passes a stationary registration pin on the stator. Each loop is developed continuously in a rotor gap by a driven input sprocket as the gap travels from a film inlet location to a projection aperture. The loop then decays continuously as it travels from the aperture to a driven output sprocket. The registration pin is located adjacent the aperture for locating the film during projection.
A number of improvements in the original Jones mechanism are disclosed in U.S. Pat. Nos. 3,600,073; 4,365,877; and 4,441,796 (all to Shaw).
Generally speaking, existing rolling loop projectors are relatively large and complex, and hence expensive to make. For example, the rotor may have an overall diameter of about 40 inches (10.sup.16 millimeters) in the case of a projector for a 15 perforation 70 mm film format. For other formats, the same considerations apply but the actual size varies. As such, the projectors also take up significant floor space, i.e. require large projection rooms in theatres. Typically (though not necessarily), the rotor of a rolling loop projector rotates in a horizontal plane (about a vertical axis) and sufficient space must be allowed for ancillary equipment such as film platters and other ancillary devices as well as for access to the projector by the operator and maintenance personnel.
A projector normally includes a so-called "field flattener" lens element at the position of the aperture, on which successive frames of the film are laid for projection, as the film is advanced through the projector (see for example U.S. Pat. Nos. 4,365,877 and 4,441,796 (Shaw)). The optical characteristics of the field flattener lens element are quite critical to proper projection. In particular, the face of the field flattener lens element on which the film is laid must have a precise shape. Traditionally, that face is defined by an arc centered on the rotational axis of the rotor, and the radius of the rotor is selected to match the curvature of the lens element face so as to maintain a uniform gap between the lens element and the rotor. Precise control and positioning of the film at the point of projection is of course critical to projected image quality and it was thought that the provision of a uniform gap between the field flattener and the rotor was essential to achieve this. As a result, it was believed that the radius of the rotor could not be reduced below a minimum determined by the curvature of the field flattener face.